The present invention relates to an electron lens assembly employed in electron microscopes and others. More particularly, the invention is concerned with an electron lens assembly of an ultra-high vacuum structure.
In general, in order that a specimen surface is to be observed in a clean state or environment within the electron microscope, it is necessary to sustain a vacuum on the order of 10.sup.-10 Torr within a space in which a specimen is placed. To this end, a so-called dry vacuum pump such as ion pump, turbo molecular pump or the like must naturally be used as the pump for generating vacuum. Further, such a column structure is required in which vacuum is generated only in the region constituting the passage for an electron beam. More specifically, the column has to be realized in such a structure in which outgassing sources such as electron lenses, deflecting coils and others are installed outside of the vacuum space by using a lining tube. Besides, in precedence to the use of the electron microscope, it is indispensable to heat the evacuated portions for the purpose of de-gassing H.sub.2 O or other molecules therefrom.
Except for the objective lens, all structural components of other electron lenses inclusive of magnetic pole pieces can be installed outside of the vacuum space in a relatively simple arrangement since there is no necessity to insert any objects in these other electron lenses externally of the microscope. In contrast, in the case of the electron lens to serve as the objective lens, a specimen holder inclusive of a specimen, an objective aperture, cold fingers and others must be inserted between the magnetic pole pieces constituting parts of the objective lens. Thus, the objective lens is necessarily of a much complicated structure and presents many difficulties in creating the vacuum environment when compared with the other electron lenses.
FIG. 1 of the attached drawings is a cross-sectional view showing a hitherto known structure of the objective lens assembly, wherein reference numeral 1 denotes an upper yoke member, 2 denotes a lower yoke member, 3 denotes an exciting coil, 4 denotes an upper magnetic pole piece, 5 denotes a lower magnetic pole piece, 6 denotes a coupling member of a non-magnetic material for combining together the magnetic pole pieces 4 and 5 in an integral structure, and 7 denotes a lens hold-down member. The elements 1, 2, 4, 5 and 7 cooperate to constitute a magnetic circuit, wherein a magnetic field is generated between the magnetic pole pieces 4 and 5, which serves as an electron lens. A specimen 8 is disposed between the magnetic pole pieces 4 and 5. The lens hold-down member 7 and the magnetic pole pieces 4 and 5 have respective bores formed therein to allow an electron beam to pass therethrough. The lower yoke member 7 is of a cylindrical structure having a hollow interior or through-hole for allowing the electron beam having passed through the electron lens constituted by the magnetic pole pieces 4 and 5 to pass through the yoke member 7. In operation, the electron beam enters the objective lens structure from above the lens hold-down member 7 and passes through an electron beam passage pipe 9 disposed within the hollow space defined by the through-hole to run to the succeeding stage of electron lens. Reference numeral 10 denotes a spacer formed of a non-magnetic material and provided with transverse bores 11 and 11' through which a specimen holder, objective aperture and others can be inserted. Reference numeral 12 denotes an O-ring for coupling the upper yoke member 1 to a portion of the column (not shown) to be disposed on the yoke member 1 in a vacuum-tight manner, 13 denotes an O-ring for joining vacuum-tightly the lower yoke member 2 and the spacer 10, and numerals 14 and 15 denote vacuum-tightly sealing O-rings for allowing an astigmatism correction element 16 to be disposed at a location close to the lower magnetic pole piece 5. Reference numeral 17 denotes a weld for joining vacuum-tightly the upper yoke member 1 and the spacer 10 by Heliarc welding. Accordingly, the upper yoke member 1 and the spacer 10 are realized in an integral structure such that they cannot be mechanically separated from each other. In contrast, the vacuum-tight connection between the lower yoke member 2 and the spacer 10 is assured by an O-ring seal 13, because it is necessary that the upper yoke member 1 and the lower yoke member 2 can be separated for accommodating the exciting coil 3.
As will be seen from the above description, the vacuum tightness of the prior known electron lens assembly at the locations in the vicinity of the specimen is supposed to be assured by using the O-ring seals 13 to 15 which are each formed of a high molecular material having a melting point of about 150.degree. C. such as fluorine rubber commercially available under the designation "Byton" or the like. Accordingly, these O-ring seals are incapable of withstanding the heating at a temperature of about 200.degree. C. which is required for realizing an ultra-high vacuum on the order of 10.sup.-10 Torr. Further, the air may enter the electron lens assembly through these O-ring seals of the rubber material. Thus, the electron lens of the structure known heretofore suffers a problem that the ultra-high vacuum can not be attained.
The inventors of the present invention have precedently proposed in JP-A-No. 60-158539 an objective lens structure in which these O-ring seals are eliminated. The objective lens structure according to this precedent proposal is shown in FIG. 2 of the accompanying drawings.
Referring to FIG. 2, a gap is formed between a lower magnetic pole piece 5 and a lower yoke member 2, wherein a spacer 10 is so disposed as to extend into a space within the lower yoke member 2 by taking advantage of the gap so that the spacer can be connected to an electron beam path defining pipe 9 by welding. This structure is based on the results of experiments conducted by the inventors which showed that when the gap is at most about a tenth part of the inter-pole distance between the upper and lower magnetic pole pieces, no significant adverse influence is exerted to performance of the electron beam by the gap defined between the lower magnetic pole piece 5 and the lower yoke member 2. More specifically, since the inter-pole distance is about 10 mm, the distance between the lower magnetic pole piece 5 and the lower yoke member 2 can be held less than 1/10 of the inter-pole distance, provided that the extension of the spacer 10 has a thickness not greater than 0.5 mm.
However, inspection conducted later on has proven that the thickness of the extension of the spacer on the order of 0.5 mm is inadequate for attaining the vacuum of 10.sup.-10 Torr because of penetration of the air or other gases. Further, it is a matter of cause that it is most preferable that there is provided no such gap.